Is this the second Boston Marathon bomb?

DETECTIVES hunting the Boston bomber were closing in on a suspect last night after finding video footage of a man apparently planting a device.

A pale bag goes unnoticed by crowds moments before the second explosion

Security film from a department store showed a grainy figure placing a bag next to a postbox in front of crowds near to the finishing line of the Boston Marathon.

It is believed to contain the second of two bombs which killed three spectators and left another 176 injured.

Forensic experts were enhancing the pictures to get a clearer image of the individual.

Reports last night that a suspect had been arrested were later denied.

Yesterday’s dramatic breakthrough came after FBI officers examined video images including some shot by a local television station.

The TV footage shows a pale bag by the postbox, believed to be the same one planted by the man in the department store security film.

The pictures show a bag that is seemingly abandoned after the man has left, on the outside of the security barriers lining the street. Crowds of spectators are packed behind the barriers, but no one is paying attention to the bag.

Moments later, another picture shows the same site – but now it is one of devastation.

The barriers have been blown apart and one is mangled around the mailbox. Spectators have scattered in panic except for some who were apparently gravely injured.

TV pictures blurred out some of the details because of the victims’ horrific injuries.

The two crude bombs which caused the devastation were made from pressure cookers.

The scene immediately following the second blast

There is no current indication to suggest that the events in Boston are indicative of a broader plot.

Homeland Security Secretary Janet Napolitano

Officers said yesterday that the lid from one of the partially destroyed devices had been found on a rooftop and was last night being regarded as a major clue.

Meanwhile White House security yesterday intercepted a suspicious letter addressed to President Obama and believed to contain ricin which is 1,000 times more toxic than cyanide.

The letter was undergoing tests last night and was being treated in the same manner as a separate one also laced with ricin and sent to Republican senator Roger Wicker.

The US Secret Service said it intercepted the letter which contained a “suspicious substance” at a mail facility away from the White House.

American authorities said they believe the same suspect is behind both letters and that there is no connection with the Boston attack.

Homeland Security Secretary Janet Napolitano hinted that the Boston attack could be the work of a lone wolf terrorist, either home-grown or foreign.

She said: “There is no current indication to suggest that the events in Boston are indicative of a broader plot.”

Investigators yesterday continued to appeal for people who had been filming and photographing the marathon to come forward. They believe their images could hold more vital clues.

Senator Angus King said: “There is probably a photograph of the bomber on someone’s iPhone.”